The invention relates to conveyor belts and, more particularly, to modular plastic conveyor belts constructed of plastic belt modules hingedly interlinked by tapered oblong hinge pins.
Because they do not corrode and are easy to clean, plastic conveyor belts are used widely, especially to convey food products. Modular plastic conveyor belts are made up of molded plastic modular links, or belt modules, arranged in rows. Spaced apart link ends extending from each end of the modules include aligned apertures to accommodate a pivot rod. The link ends along one end of a row of modules are interleaved with the link ends of an adjacent row. A pivot rod, or hinge pin, journalled in the aligned apertures of the end-to-end-connected rows, connects adjacent rows together to form a two-ended belt or an endless conveyor belt capable of articulating about a drive sprocket.
In many industrial applications, conveyor belts are used to carry products along paths including curved, as well as straight, segments. Belts capable of flexing sidewise to follow curved paths are referred to as side-flexing, turn, or radius belts. As a radius belt negotiates a turn, the belt must fan out because the edge of the belt at the outside of the turn follows a longer path than the edge at the inside of the turn. To enable the belt to fan out, the apertures in the link ends on one end of each row are typically elongated in the direction of belt travel. The elongated apertures allow the belt to collapse at the inside of a turn and to spread at the outside.
The requirement of following a curved path causes problems not found in straight-running belts. For example, because the elongated apertures of conventional radius belts are identical in length across the width of the belt, only one or a very few of the link ends at the outside of a turn bear the entire belt pull. On a straight run, the belt pull is distributed across the entire width of the belt. Unless the outer link ends are specially bolstered, the belt pull strength rating is limited by the pull strength in a turn, which is often up to ten times less than on a straight. Thus, radius belts must be heavier and stronger than straight-running belts conveying the same load.
A conveyor belt having special edge modules with closer link end spacing and tapered pivot rod slots for circular pivot rods to improve the distribution of the pull at the outside of a turn is disclosed in U.S. Pat. No. 5,164,139, issued Dec. 29, 1992. The patent also discloses the use of a semi-tapered circular rod with untapered pivot rod slots to achieve a similar effect. A belt made up of those edge modules, however, still confines the belt pull in a turn to only a few closely spaced, thin link ends at the outside of the turn. The belt""s strength in turns is less than on straight runs. The disparity in strength is greater the wider the belt. Thus, belt strength must be wasted to accommodate turns.
A radius belt that uses a rotatable tapered link shaft is described in U.S. Pat. No. 5,431,275, issued Jul. 11, 1995. The link shafts have a longitudinal axis, a straight surface for carrying the tensile load between adjacent rows in a turn. The tapered surface is disposed angularly about the longitudinal axis relative to the straight surface. The tapered surface intersects the straight surface so as to form a transition zone along which the tensile load on the straight surface is rotatably transferred to the tapered surface when the conveyor belt changes from traveling on a straight path to a radial path. Such a link shaft has complex outer surfaces and must not be impeded by dirt and debris from rotating properly between surfaces to be effective in transferring load as the belt goes from a straight to a curved path. The patent also discloses the use of tapered shaft apertures to help share tensile loads, but such apertures are difficult to mold.
A sought-after feature in radius belts is a low turning ratio, i.e., the ratio of the radius of the tightest conveyor turn path to the width of the belt. Most radius belts have turning ratios of about 2:1 or greater. Thus, turns must be long and gradual, taking up otherwise usable space. Smaller turning ratios are generally limited by interference between the interleaved link ends as they collapse at the inside of a turn. Conventional line ends are formed along parallel, straight axes and tend to bind in turns. A dual-pitch belt that collapses better at the inside of a turn disclosed in U.S. Pat. No. 5,346,059, issued Sep. 13, 1994. The belt shown has shorter link ends on the inside half of the belt than on the outside half, which allows the inside edge to collapse tighter. The pivot rod apertures along each half, however, are slotted in transverse alignment with one another, and the load is borne by only the outermost and centermost link ends in a turn. On a straight run, belt pull is shared among many link ends. Consequently, the belt must be made much stronger or its load derated in order to handle the turns.
Wasted belt strength, large differences in pull strength on straights and turns, complex provided designs, molding difficulties, and other shortcomings are avoided by the invention, which provides a modular conveyor belt capable of following straight or curved paths. The belt is constructed of a succession of rows of belt modules having hinge elements projecting from each end and spaced apart along the width of the row. The forward hinge elements of a row are interleaved with the rearward hinge elements of an adjacent row. Openings in the interleaved hinge elements are generally aligned to form a passageway across the width of the rows to accommodate, preferably, a tapered oblong hinge pin, which forms a hinge between adjacent rows.
In a preferred version of the belt, the hinge pin includes a first region and a second region. The first region extends from a first end of the pin toward an opposite second end. The second region extends from the second end toward the first region. The cross section of the hinge pin over the first region is oblong with a long axis in the direction of belt travel having a constant length. The cross section of the hinge pin over the second region is generally oblong, but with the length of the long axis of the cross section increasing with distance from the second end to a maximum length less than or equal to the length of the constant long axis in the first region. A belt using such hinge pins thus enjoys the advantages of variable pitch.
In another version, the hinge pin includes a third transition region between the first and second regions, also oblong in cross section in which the length of the long axis of the cross section in the third region varies from the constant long axis of cross section in the first region to the maximum long axis of the cross section in the second region to provide a smooth transition from straight to curved conveying paths and vice versa. The oblong hinge pin with the transition region allows belt tensile loads to be shared among a number of hinge elements in turns, on straights, and in transitions between turns and straights. In this way, the belt does not have to be oversized just to enable it to handle turns.
In a preferred version of the belt, the hinge elements of consecutive rows have different-shaped openings. In a first row, the openings in the hinge elements at both ends are elongated oblong slots having dimensions greater than the maximum cross-section dimensions of the hinge pin. The length of the elongation is greater than the oblong hinge pin""s constant cross-sectional long axis to allow the belt to fan out at the outside of a turn and collapse at the inside of a turn. In a second row of a preferred version, the openings in the hinge elements at each end are in the form of fan-shaped sectors having a radius slightly greater than the greatest dimension of the hinge pin""s cross section. The vertex of the fan-shaped apertures is toward the distal ends of the hinge elements. The fan shape accommodates the oblong hinge pin and allows the belt to back flex and to flex forward to articulate about a drive sprocket or idler.
Preferably, for belts used to negotiate curves in one direction only, the hinge elements are curved slightly along the turning radius to provide better strength in a turn and to enable a smaller turning radius without unduly stressing the ends of the hinge elements.
In yet another version of the belt, each row comprises a series of generally identical links disposed side by side. Each link comprises a hinge element at either end extending from a central transverse element. The transverse element has a central bore parallel to the openings in the hinge elements. The central bore admits a support element, such as a threaded rod or bolt, on which a plurality of links can be stacked side by side to form a belt row. The transverse elements act as spacers to allow spaces between consecutive hinge elements to accommodate the hinge elements of an adjacent row. Fastener hardware at each side of the row engage that ends of the bolt and hold the stack of links together. The individual links allow custom belts of different widths to be made simply.
The individual link design also makes it easier to add various other accessories and features to the belt by incorporating the features into special links. For example, links having depending drive lugs for an intermediate drive, or side-projecting gear teeth for engaging a horizontal gear wheel, or depending holddown guides can be formed on individual links and used where needed. Furthermore, special belt edge links at the side edges of each row could be used to provide recesses for the fastener hardware for the bolt so that protrusions are eliminated and the side edge of the belt is less likely to snag objects is passes. Thus, the individual links allow for a custom belt to be made with only a small collection of interfitting parts.